<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .bar {
            width: 150px;
            height: 15px;
            border: 1px solid red;
            border-radius: 5px;
            padding: 1px;
        }

        .bar_in {
            width: 49%;
            height: 100%;
            background-color: blue;
            /*添加过渡效果*/
            transition: all 1s ease;

        }

        .bar:hover .bar_in {
            width: 100%;
        }
    </style>
</head>

<body>
    <div class="bar">
        <div class="bar_in"></div>
    </div>
</body>

</html>